1. Field of the Invention
This invention relates to integrated circuit (I.C.) capacitors, and more particularly to interdigitated capacitors fabricated on an I.C. die.
2. Description of the Related Art
Capacitors, used in a vast number of electronic circuits, exist in both discrete and integrated forms. Discrete capacitors can provide a relatively large, precise value of capacitance. Many circuits, however, are preferably implemented as I.C.s, where a discrete capaci-or is of no use. A discrete capacitor may be used as part of a hybrid circuit, but this can be costly, requires a larger package and an intricate installation step, and necessitates the use of electrical interconnections such as wire bonds, which tend to degrade a circuit's performance, particularly at higher frequencies.
Integrated capacitors, on the other hand, may be fabricated on an I.C. die simultaneously with other circuit components, and interconnections to them are made with the same I.C. metallization layer(s) used to interconnect the other components. Thus, standard integrated capacitors are obtained at virtually no cost in terms of I.C. processing steps. They do suffer from a major shortcoming, however, in that the value of capacitance which can be practically obtained is very small, due to their small size. The capacitance C of a capacitor formed from two parallel conductive plates is given by: EQU C=.di-elect cons.A/d, Eq. 1
where A is the "active area", i.e., the area described by the overlap of the two plates, d is the separation between the plates, and .di-elect cons. is the permittivity of the dielectric material between the plates. As an I.C.'s die size increases, its cost tends to rise and its yield tends to fall. As a result, die sizes are typically kept as small as practically possible, and the amount of die area allocated for capacitors is very limited.
In view of the unrelenting push to further reduce the size of electrical circuits, a need exists for an integrated capacitor which can provide a higher capacitance per unit area than is currently available.